1. Field of the Invention
The present invention relates to a laser-light source which uses optical fibers and optically multiplexes a plurality of laser beams emitted from a plurality of semiconductor lasers.
The present invention also relates to an exposure system which uses the above laser-light source as a light source for exposure.
2. Description of the Related Art
Conventionally, in order to generate a laser beam having a ultraviolet wavelength, wavelength conversion lasers, excimer lasers, and Ar lasers have been used in practice. In the wavelength conversion lasers, infrared light emitted from a solid-state laser excited with a semiconductor laser is converted into a third harmonic having an ultraviolet wavelength.
Further, recently, GaN-based compound semiconductor lasers which emit a laser beam having a wavelength in the vicinity of 400 nm have been provided, for example, as disclosed in Japanese Journal of Applied Physics Part 2 Letters, vol. 37, 1998, pp. L1020.
Light sources which emit laser beams having the wavelengths as mentioned above are being considered for use in exposure systems for exposure of photosensitive materials which are sensitive to light in a predetermined wavelength range including an ultraviolet wavelength range of 350 to 420 nm. In such a case, the light sources for exposure are required to have sufficient output power for exposing the photosensitive materials. The above predetermined wavelength range is hereinafter referred to as the ultraviolet range.
However, the excimer lasers are large in size, and the manufacturing costs and maintenance costs of the excimer lasers are high.
In the wavelength conversion lasers which convert infrared light into a third harmonic in the ultraviolet range, the wavelength conversion efficiency is very low. Therefore, it is very difficult to achieve high output power. In a typical wavelength conversion laser at the currently practical level, a solid-state laser medium is excited with a semiconductor laser having an output power of 30 W so as to output a fundamental harmonic having a wavelength of 1,064 nm and an output power of 10 W, the fundamental harmonic is converted into a second harmonic having a wavelength of 532 nm and an output power of 3 W, and a third harmonic having a wavelength of 355 nm (i.e., a sum frequency of the first and second harmonics) and an output power of 1 W is obtained. In this wavelength conversion laser, the efficiency in electric-to-optical conversion in the semiconductor laser is about 50%, and the efficiency in conversion to the ultraviolet light is as low as about 1.7%. In addition, since an optical wavelength conversion element, which is expensive, is used in the above wavelength conversion laser, the manufacturing cost of the wavelength conversion laser is high.
Further, the efficiency in electric-to-optical conversion in the Ar lasers is as low as 0.005%, and the lifetime is as short as about 1,000 hours.
On the other hand, since it is difficult to obtain a low-dislocation GaN crystal substrate, an attempt has been made to achieve high output power and reliability in a GaN-based compound semiconductor laser. In the attempt, a low-dislocation region having a width of about 5 micrometers is produced by a growth method called ELOG (epitaxial lateral overgrowth), and a laser region is formed on the low-dislocation region. However, even in this attempt, it is difficult to obtain a low-dislocation substrate having a large area. Therefore, no GaN-based compound semiconductor laser having a high output power of 500 mW to 1 W has yet been commercialized.
In another attempt which is being considered for increasing output power of a semiconductor laser, for example, a hundred cavities each of which outputs light with an output power of 100 mW are formed so as to obtain a total output power of 10 W. However, it is almost unrealistic to manufacture as many as a hundred cavities with high yield. In particular, it is difficult to manufacture GaN-based compound semiconductor lasers each having many cavities since manufacture of GaN-based compound semiconductor lasers with a high yield of 99% or greater is difficult even in the case of single-cavity GaN-based compound semiconductor lasers.